Conventionally, aluminium is produced by the electrolysis of alumina dissolved in a cryolite based molten salt bath in the more than a hundred years old Hall-Heroult process. In this process carbon electrodes are used, where the carbon anode taking part in the cell reaction resulting in the simultaneous production of CO2 and aluminium according to the equation:2Al2O3+3C=4Al+3CO2  (1)
The carbon anodes of today's cells are consumed in the process with a gross anode consumption of some 500 to 550 kg of carbon per ton aluminium produced. The use of carbon anodes results in the production of pollutant greenhouse gases like CO and CO2 in addition to the so-called PFC gases (CF4, C2F6, etc.).
Edwards, L. and Kvande, H.: “Inert anodes and other technology changes in the aluminium industry—The benefits Challanges, and impact on present technology”, JOM, 28-33, May, 2001, have estimated the specific CO2-equivalent emissions from various production process for todays average aluminium electrolysis cells with carbon anodes. Omitting the CO2 emissions from the power production processes as well as from bauxite mining and alumina production, Edwards and Kvandes data show that the world average emissions equals some 3.7 tonne CO2-equivalents per tonne of aluminium produced. New, modern aluminium smelters with prebake technology erected today usually have annual capacities in the range 230-260 kt aluminium. For such smelters, the emissions of CO2 and PFCs from anode production and electrolysis usually is approx. 1.8 tonne CO2-equivalents per tonne of aluminium produced, i.e. amounting to some 410-470 kt CO2-equivalents anually. The demonstrated gap between BAT electrolysis data and the world average data is of course due to the high tonnages of aluminium produced in old plants with poorer emission control (i.e. Søderberg plants).
U.S. Pat. No. 6,117,302 discloses a method and an apparatus for electrolytically smelting alumina to produce aluminium metal, where a solide oxide fuel cell and an electrolytic smelting cell are combined in an integrated manner. In one embodiment, oxygen gas evolved at one inert anode in said smelting cell is allowed to flow to the cathode side of the fuel cell where it is reduced. In one aspect, an alumina ore refinery for producing refined alumina is positioned near the solid oxide fuel cell. One advantage with this combination is that heat generated in one part of the apparatus may be exploited in an other part of it.
Due to the consumption of the carbon anode and the electrolysis process causing emissions of greenhouse gases as CO2 and fluorocarbon compounds, the replacement of carbon anodes with an effectively inert material would be highly advantageous for both cost and environmental reasons. With a dimensionally stable, oxygen evolving anode (i.e. an inert anode), used in the electrowinning of aluminium oxide, the net reaction would be:2Al2O3=2Al+3O2  (2)
This means that a 250 kt aluminium primary production plant would emit some 450 kt of oxygen. The produced oxygen gas can be collected in the off-gas system and purified with respect to removal of dust, electrolyte particles and fluoride vapours. The produced oxygen then represents a commercial value, and can be compressed and sold as liquid/pressurised oxygen in an existing world marked. The economic value of 450 kt oxygen from a 250 kt aluminium plant would be in the order of 10-15 million US$. The mentioned volume of oxygen, however, is most likely to large for the cylinder market and only world scale production units can make use of such a large amount of oxygen. This probably require an oxygen consuming large scale production plant (e.g. methanol plant, GTL plant, steel production plant, power generation plant, etc.) close to the aluminium plant in order to make use of the oxygen. Therefore, it seems more economically and technically feasible to utilise the produced and purified oxygen on site (, i.e. omitting the cost accompanied by construction a liquid oxygen production facility).
An aluminium production plant would require a substantial amount of electric power. A 250 kt aluminium primary production plant would need about 340 cells with a cellvoltage each of 4.2 V and an amperage of 260 kA, if a current efficiency of 96% is assumed. Thus approximately a 370-400 MW power plant is needed to supply electricity to the aluminium plant.
The main object of the present invention was to arrive at an improved process for manufacturing aluminium.
Another object of the present invention was to manufacture aluminium in a way that implies reduced effluent of pollutants.
Furthermore, another object of the present invention was to arrive at a method for utilisation of oxygen that are generated in the said aluminium process for improving the performance of the electric power generating process and to significantly reduce or eliminate the effluent from said integrated power generating process.
One problem the inventors faced in their search for an improved aluminium process with reduced emissions, was that the aluminium process needed a significant amount of electric power. Conventional natural gas based power generating processes could not be adopted due to significant emissions of both CO2 and NOx from these processes. In many countries hydroelectric power or other none CO2 emitting power generation processes is not available, too expensive, or is already exploited.
Furthermore, it would be advantageous to be able to utilise oxygen generated in the aluminium process to improve the performance and reduce the cost of the applied power generation plant.
Furthermore, since the integrated aluminium and power generating plant both (in most cases) needs imported Al2O3 and fossil fuel and that CO2 must be exported to a geological formation for deposition, location close to a gas terminal, harbour or deposition area may be essential.
It would also be advantageous to make use of gases that can be used or is generated in the power plant to improve the operation of the aluminium process. Thus, the inventors started to look for solutions that might meet all these requirements.
In conventional power generating processes fuelled with a methane containing fuel, carbon dioxide and water (steam) will be produced according to the reaction:CH4+2O2=CO2+2H2O
Other hydrocarbons will produce CO2 and H2O according to the reaction:
CmHn+((4m+n)/4)*O2=mCO2+(n/2)H2O, m and n being the number of atoms of carbon and net hydrogen, respectively.
The fuel to electric power efficiency will be typically 55% based on the low heating value of the fuel gas. An integrated new inert anode based aluminium plant and fossil fuel based powerplant thus will emit about 1.2 million tons of CO2 per year.
Due to the fact that air is used as oxidant in the combustion process the CO2 in the exhaust gas from the power plant are diluted with nitrogen. Exhaust gas emitted from a natural gas fuelled combined gasturbine and steam cycle power plant contains e.g. only about 4% CO2. The exhaust gas will also contain harmful nitric oxides. Nitric oxides are generated at high temperature in the gasturbine combustion chamber due to the presence of nitrogen in the oxidant i.e. air.
Due to the environmental aspects of NOx and CO2 it is crucial that the emission of these components to the atmosphere is considerably reduced.
One method to reduce the CO2 emission is to improve the efficiency of the power generating process, but in order to achieve a significant reduction of CO2, this will not be sufficient. Another method is therefore to separate the CO2 from the exhaust gas stream, compress the CO2 and deposit the CO2 in e.g. empty oil and gas reservoirs, in aquifers or use the CO2 for enhanced oil recover or for recovery of methane from deep coil beds.
In order to meet national NOx control requirements different methods can be used for instance burner modifications, applications of catalytic burners, steam additions or selective catalytic reduction (SCR) of the NOx in the exhaust gas. Ordinary air used in combustion processes contains about 78% by volume of nitrogen. Some of the nitrogen is oxidised during the combustion to NO, NO2 and N2O (referred to as thermal NOx). At least 80-98% of the NOx formed arises from the said oxidation of nitrogen in air. The rest arises from oxidation of the nitrogen content in the fuel. Lowering the peak combustion temperature is a very effective means of reducing the amount of NOx formed. Unfortunately this causes a substantial efficiency drop due to poor combustion or due to reduced temperature in the combustion chamber in a gas turbine system. SCR (Selective Catalytic reduction) is an efficient method in reducing the NOx, but require a reduction agent such as ammonia and an expensive catalyst installed downstream the combustion process. Formation of NOx will also be significantly reduced or eliminated if the fuel is combusted with pure oxygen.
CO2 can be removed from exhaust gas, normally discharged off at near atmospheric pressure, by means of several separation processes, e.g. chemical active separation processes, physical absorption processes, adsorption by molecular sieves, membrane separation and cryogenic techniques. Chemical absorption, for instance by means of alkanole amines, is an widely discussed method to separate CO2 from exhaust gas. These separation processes, however, require heavy and voluminous equipment and will consume a substantial amount of heat produced in the combustion process. Applied in connection with a power generating process, these separation processes will reduce the power output with 10-15%. This is mainly due to the low concentration of CO2 in the exhaust gas.
An increasement of the concentration of CO2 in the exhaust gas is, however, possible by burning the carbon containing fuel with pure oxygen instead of air. Another advantage of this is that the generation of nitric oxides is almost eliminated as described above.
Commercial air separation methods (e.g. cryogenic separation or pressure swing absorption (PSA) applied for producing pure oxygen require 250 to 300 KWh/ton oxygen produced. If these methods are used for supplying oxygen to a combustion process in a gas turbine cycle these methods will reduce the net power output from the gas turbine cycle by at least 15%. The expenses of producing oxygen in a cryogenic unit will increase the price of electric power substantially and may amount to as much as 50% of the cost of the electric power.
Therefore, one method of particular interest would be to exploit the purified oxygen from the aluminium electrolysis cells directly as a feed stock for an electric power plant based on natural gas or other fossil fuels to further reduce the emission of CO2 or other harmful compounds to the atmosphere from a integrated aluminium and power generation plant.
As shown above a world scale aluminium plant applying inert anodes will generate about 1250 ton oxygen/day sufficient for a 100 MW power plant. The aluminium plant itself will consume about 370 MW electric power. This means that about 25% of the power may be generated by utilising oxygen from the aluminium process. This also implies that if the generated oxygen is utilised in an oxyfuel based power generation process as described in e.g. patent application WO 99/63210, tentatively 25% of CO2 from natural gas fired power production can be recovered for deposition. The generation of nitric oxides may also be reduced in the same order of magnitude. This solution, however, require design of a rather small oxyfuel plant of about 100 MW and one large conventional power plant.
In order to eliminate the emission of CO2 and NOx from an integrated aluminium and fossil fuel based power plant about 5000 t oxygen per day is required. An additional amount of 3270 t oxygen per day is therefore needed. Due to the high cost of oxygen produced in a cryogenic or PSA process, application of these air separation techniques is not an attractive solution.
A less energy demanding method than the cryogenic separation methods is known from the European patent application 0658 367-A2. The patent application describes an application of a mixed conducting membrane integrated with a gas turbine system and where the membrane separates oxygen from a heated air stream.
Pure oxygen near atmospheric pressure or below and at high temperature is recovered from the permeate side of the mixed conducting membrane. An oxygen partial pressure difference causes oxygen to be transported through the membrane by reduction of oxygen on the high oxygen partial pressure side (rententate side) and oxidation of oxygen ions to oxygen gas on the low oxygen partial pressure side (the permeate side). In the bulk of the membrane oxygen ions are transported by a diffusion process. Simultaneously the electrons flow from the permeate side back to the feed side of the membrane. The disadvantage of this method is that oxygen is recovered at low pressure while oxygen is needed at high pressure in the combustion process. Cooling and recompression of the recovered oxygen stream necessitate application of expensive process equipment. Recompression will also need mechanical or electrical energy that will reduce the total plant energy efficiency.
A more energy efficient method is known from Norwegian patent application NO-A-972632 (published 07.12.98). This reference describes a power and heat generating process where a fuel is combusted with an oxidant, which is an O2/CO2/H2O-containing gaseous mixture, which is supplied from a mixed conducting membrane. The oxygen is picked up from the permeate side of the mixed conducting membrane by means of a sweep gas. The sweep gas is the product or part of the product from at least one combustion process upstream the membrane. In this patent application, the sweep gas, or part of the sweep gas, containing a mixture of mainly CO2 and H2O, also act as the working fluid in a gas turbine cycle. The amount of sweep gas is related to the amount of working fluid required in the gas turbine cycle i.e. to control the temperature in the gas turbine combuster.
In Swedish patent application SE-A-0002037 the CO2 cycle compressor is omitted and a large fraction of the power is generated in a conventional air cycle gas turbine. In this case natural gas is combusted on the permeate side of a mixed conducting membrane and a CO2 and H2O containing gas mixture is produced. A major part of the heat generated during this combustion process is transferred to the air side in order to heat the air from the gas turbine compressor. Hot air then enter the gas turbine expander to generate electric or mechanical power. The generated hot CO2/H2O mixture can be cooled in order to condense water. Thus CO2 can be recovered at high pressure. Alternatively the CO2/H2O gas can be depressurized in an expander to produce electric or mechanical power. In this case CO2 will be recovered at low pressure.
Alternatively also described in patent application SE-A-0002037, the mixed conducting membrane or part of the mixed conducting membrane is replaced by a solid oxide fuel cell made of ceramic materials that only conduct oxygen and not electrons. Direct Current then can be produced along with Alternating Current from the gas turbine generator.
By application of oxygen in the power generating process, the exhaust gas from the combustion process will have a high concentration of CO2 and water and generation of nitric oxides are avoided. If water are removed by means of condensation, dry CO2 can be recovered and deposited into a geological formation.
After having evaluated various ways of generating power to a low emission aluminium process, the inventors decided to further investigate integration of the said aluminium process with a mixed conducting membrane based power generation process or alternatively including a solid oxide fuel cell based power generation process.
One requirement was that the oxygen from the aluminium plant should improve the performance of the low emission power generating plant. In Swedish patent application SE-A-0002037, all oxygen utilised in the combustion process are separated by means of a mixed conducting membrane or by means of a pure oxygen conducting membrane. If 25% of the oxygen produced by the membrane is replaced by oxygen generated in the aluminium process, the total membrane area consequently will be reduced with 25%.
It was, however, found that the addition of oxygen to the membrane based power generation process could reduce the membrane area with more than 25%. The driving force for transportation of oxygen through the mixed conducting membrane is the difference in the partial pressure of oxygen between the retentate side and the permeate side. Thus, if oxygen recovered from the aluminium process is added to the combustion process less oxygen needs to be extracted from the air stream. This means that the average partial pressure of oxygen on the air side will increase, assuming the same turbine inlet temperature in both cases to maintain gas turbine efficiency. This will increase the transport of oxygen per square meter through the membrane which will further reduce the size and cost of said membrane.
In Swedish patent application SE-A-0002037 compressed hot CO2 and H2O containing exhaust gas enter a purge gas turbine to recover heat as mechanical power. If the compressed hot exhaust gas is mixed with fuel and recycled CO2 and oxygen from the aluminium plant and further combusted, this will increase the power generation in the purge gas turbine. Since more power is generated in the purge gas turbine less power needs to be generated in the main gas turbine, also reducing the total membrane area with about 25%. Since about 25% less energy needs to be transferred to the air stream, the total heat exchanger area also will be reduced with about 25%.
If a solid oxide fuel cell is integrated with the power generation process less than 80% of the fuel added to the fuel cell will be converted or combusted. Oxygen recovered from the aluminium process then can be used to combust unconverted gas to CO2 and water. Heat from the combustion of unconverted fuel can be used to pre-heat air to the solid oxide fuel cell.
Another requirement was that gas generated in the power generation process should improve the operation of the aluminium process. In order to remove oxygen generated at the aluminium anodes, application of a sweep gas would be preferable. Since pure oxygen is very aggressive against most materials it is an advantage to dilute the oxygen to a certain degree to reduce hazard risks or improve lifetime of equipment. In most cases pure oxygen also must be diluted to be used in a combustion process to control the combustion temperature. CO2 recovered from the power plant can then be used both as sweep gas in the aluminium process to improve operation and as dilutant in the combustion process in the low emission power generation process. If used as sweep gas in the aluminium process the CO2 gas, however, need to be dry without moisture. In the present invention one part of the partly dried CO2 is further dried by means of a commonly known gas drying process, and is fed to the aluminium plant to be used as anode sweepgas. The recovered mixture of CO2 and oxygen is purified with respect to removal of dust, electrolyte particles and fluoride vapours. The purified oxygen gas mixture then can be compressed to elevated pressure to be used as oxidant in the low emission power generation process.